Low-molecular-weight nematic compounds are used in a wide variety of display elements, such as watches, pocket calculators and displays, and are distinguished by good alignability and switchability in the electric field. However, the aligned structure is not particularly stable without an applied electric field.
Applications in polarizers, retarders, etc., require compounds which have good nematic, thermally stable alignment. In addition, applications in polarizers need a nematic phase in a broad temperature range, since introduction of auxiliaries (for example dyes) can cause a narrowing of the nematic phase.
A known way of fixing liquid-crystalline properties is to bind the liquid crystals into polymeric networks, by subsequent crosslinking of aligned liquid-crystalline side-chain polymers. However, liquid-crystalline side-chain polymers are in general more difficult to align than low-molecular-weight liquid crystals owing to their higher viscosity, which generally rises with molecular weight. Examples of readily alignable liquid-crystalline side-chain polymers are cholesteric siloxanes. If the cholesterol in these materials is replaced by simple compounds which favor the formation of a nematic phase (nematogenic compounds), although nematic compounds are generally obtained, they only have a small phase width and are not readily alignable.
U.S. Pat. No. 5,211,877 describes a nematic compound which is based on cyclic polysiloxanes and mesogens containing two aromatic rings and which has only a narrow nematic phase of 69.degree. and moderate alignability.